Methods of manufacturing electrical insulating structures

ABSTRACT

Methods of manufacturing insulating structures for electrical inductive apparatus, such as angular, channel-shaped structures for coil edge insulation. The methods include the step of subjecting the insulator blank to steam for a predetermined period of time, which step is promptly followed by forming the blank into the desired configuration in a heated press.

United States Patent Moore et a1.

[ June 27, 1972 METHODS OF MANUFACTURING [56] References Cited ELECTRICAL INSULATING STRUCTURES UNITED STATES PATENTS 3,058,160 10/1962 Mocker ..264/287X lnventors: Harold R. Moore, R. R. 6 Forest H1118, 00 937 Zinser l "264/287 Munciey 47302; Fischer 12261376 4/1968 Caron ..264/ll9 Centalane, Elm Grove 3,557,277 1/1971 Broderscn ..264/322X Filed: Oct 8, 1970 Primary Examiner-Robert F. White Assistant Examiner-Richard R. Kucia Att0meyA. T. Stratton, I. E. Broder and D. R. Lackey [57] ABSTRACT Methods of manufacturing insulating structures for electrical Appl. No.: 79,284

Related U.S. Application Data Continuation of Ser. No. 744,210, July 11, I968,

abandoned" inductive apparatus, such as angular, channel-shaped structures for coil edge insulation. The methods include the step of U.S. c1 ..264/l38, 264/294, 264/322 subjecting the insulator blank to steam for a predetermined Int. Cl. v B291! 3/00, B29] 5/06 period f time which step is promptly fonowed by forming the Field of Search ..264/285287, 294-296, blank m the desired configuration in a'heated press 2 Claims, 5 Drawing Figures 80 82 84 L CUTTING BLANK HEATING BLANK COR'RUGAT|NG TO SIZE WITH STEAM BLANK 86 88 HEATING BLANK FORMING WITH STEAM BLANK PATENTED UN27 1972 3,673,297

FIG.2

FIG.

70 72 74 I 2 CUTTING BLANK HEATING BLANK FORMING 4 TO SIZE WITH STEAM I BLANK 8O 82 84 I CUTTING BLANK HEATING BLANK CORRUGATING TO SIZE WITH STEAM BLANK FIG.5

HEATING BLANK FORMING WITH STEAM BLANK INVENTORS WITNESSES Harold R. Mqore 8 Heinz G. Fischer METHODS OF MANUFACTURING ELECTRICAL INSULATING STRUCTURES This is a continuation of application Ser. No. 744,210 filed July 1 1, 1968 and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates in general to methods of manufacturing insulating structures for electrical inductive apparatus, such as transformers and reactors, and more particularly to methods of forming angular, channel-shaped insulators for fitting the edges of coils used in such apparatus.

2. Description of the Prior Art Electrical inductive apparatus, such as transformers and reactors of the shell-form type, require the manufacture of difficult to form, angular, channel-shaped structures which fit over the inner and outer edges of each of the plurality of pancake coils which make up the winding, or windings, of the apparatus. The channel shaped insulators are preferably formed from a fairly thick sheet of insulating material, such as 0.125 inch thick pressboard.

Prior art methods of manufacturing channel insulating members for the outer edge of pancake coils are either slow, and therefore costly, or they result in a product which is less than optimum. For example, some methods utilize pressboard having a high rag content, with the insulator blank being cut to shape, soaked in water, and pressed to the desired configuration in a hot press. The insulator structure must be kept in the hot press until the moisture has evaporated, as premature removal of the pressure will allow any retained moisture to turn to steam, causing blisters to form in the structure.

In an attempt to reduce the time required in the press, some prior art methods merely spray the outside surface of the pressboard blank with water, and then place the blank in a heated press. While this method reduces the required press cycle time, it often results in tearing or rupturing of the pressboard fibers at the sharply bent portions of the structure, and the structure tends to loose its as formed shape due to the memory of the fibers, which attempt to return the structure to its original shape. Thus, the structure does not fit the coil edges as snugly as it should, which reduces its value as electrical insulation.

Still another method of treating pressboard prior to forming, involves corrugating, and then re-corrugating the pressboard, which method is disclosed in US. Pat. No. 3,351,693, which is assigned to the same assignee as the present application.

In addition to the hereinbefore mentioned disadvantages of the prior art methods, some complicated structures, such as the angular channel members for the inner edges of the pan-, cake coils cannot economically be formed of pressboard, requiring a more costly laminated structure formed of a plurality of plies of stretchable crepe paper, bonded together with a heat settable resin. An example of this type of construction is disclosed in US. Pat. No. 3,189,681, which is assigned to the same assignee as the present application.

It would be desirable to be able to construct the channel shaped insulating structures for both the inner and outer edges of pancake coils, as well as other complicated insulating structures, from pressboard, to form structures which retain their as formed" shape, to form structures without resorting to the more costly high rag content pressboard, to form structures without resorting to soaking the pressboard in water, which increases the press cycle time, to form structures without rupturing or tearing the fibers, and to form structures without resorting to two successive corrugating steps.

SUMMARY OF THE INVENTION Briefly, the present invention is a new and improved method of manufacturing complicated insulating structures from pressboard, such as the angular channel-like members for both the inner and outer edges of pancake coils for shell-form type inductive apparatus, which not only reduces press time to a minimum, but which results in a well defined structure free of cracks and ruptures, which retains its as formed shape.

In a first embodiment of the invention, suitable for structures such as the outer edge angular channel-like insulating members, the method comprises cutting a blank to size from a sheet of insulating material, such as pressboard, subjecting the blank to steam for a predetermined period of time, and promptly forming the blank in a press having heated dies. The steam rapidly penetrates the pressboard causing it to swell and break the bonds between the fibers of the pressboard, without absorbing an excessive amount of moisture. The blank is then formed while it still is at the elevated temperature due to the steam, which gives the insulating material extreme plasticity, allowing the press to rearrange or reorient the fibers without cracking or rupture, and without memory being retained in the fibers.

In a second embodiment of the invention, suitable for shapes heretofore not economically feasible for pressboard, the method includes a corrugating step. After the blank is subjected to steam for a predetermined period of time, at least the portion of the blank which will be subjected to extreme shaping is corrugated, to mechanically work and further soften the pressboard. The corrugating step is followed by again subjecting the blank to steam, and then the blank is promptly formed in a heated press, the same as in the first embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS Further advantages and uses of the invention will become more apparent when considered in view of the following detailed description and drawings, in which:

FIG. 1 is an elevational view, in section, of electrical inductive apparatus of the type which may utilize insulating structures constructed according to the teachings of the invention;

FIG. 2 is a perspective view of an angular, channel-like insulating structure for the outside edge of a pancake coil, which may be constructed according to the teachings of the invention;

FIG. 3 is a perspective view of an angular, channel-like insulating structure for the inner edge of a pancake coil, which may be constructed according to the teachings of the invention;

FIG. 4 is a block diagram illustrating the steps in the method of manufacturing insulating structures according to a first embodiment of the invention; and

FIG. 5 is a block diagram illustrating the steps in the method of manufacturing insulating structures according to another embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring now to the drawings, and FIG. 1 in particular, there is shown an elevational view, in section, of electrical inductive apparatus 10, such as a transformer or reactor, which may utilize insulating structures manufactured according to the methods of the invention. Specifically, inductive apparatus 10 includes a magnetic core-winding assembly 12 disposed within a tank 14, which may be filled with a suitable insulating and cooling dielectric, such as oil. The electrical bushings for bringing electrical conductors through the wall of tank 14 for connection to the windings are not shown in order to simplify the drawing.

The magnetic core-winding assembly 12 is illustrated as being of the shell-form construction, having a plurality of pancake type coils, such as pancake coil 16, disposed in inductive relation with a magnetic core 18. Magnetic core 18 has first and second similar assemblies or sections 20 and 22. Each of the magnetic core assemblies 20 and 22 are formed from a plurality of metallic, magnetic laminations 24, which are stacked to form a rectangular structure having four interconnected stacked sections arranged to define a window or opening through which the pancake coils may pass. The two assemblies 20 and 22 are disposed in side-by-side relation, with their adjacent stacked sections forming a winding leg which the pancake coils encircle.

The pancake coils, such as pancake coil M, are each constructed of a plurality of turns 26 of electrical conductor, such as copper or aluminum, with the turns being insulated and wound in a substantially rectangular configuration having rounded outer corners, and defining an opening through which the winding leg of magnetic core 18 may pass. The pancake coils are disposed in side-by-side relation, with their openings in alignment, and they are interconnected to form the winding, or windings, of the electrical inductive apparatus 10.

The inner and outer edges of the pancake coils, such as pancake coil 16, are provided with additional electrical insulation in the form of channel-like insulating structures, which should snugly fit the edges of the coils in order to efficiently utilize the insulating qualities of the structures. Further, the insulating structures should not be cracked or ruptured, as any voids in the insulation produce weak spots which may eventually breakdown under high electrical stresses.

Specifically, the outer edge of pancake coil 16 includes a plurality of channel-like insulating structures 30, 32, 34, 36, 38, 40, 42, 44, 46 and 48, which tightly fit the outer edge, and which have their ends scarfed and overlapped to form a substantially continuous insulating structure about the outer edge of the pancake coil. In like manner, the inner edge of pancake coil 16 includes a plurality of channel-like insulating structures 50, 52, 54, 56, 58, 60, 62, 64, 66 and 68, which tightly fit the inner edge, and which have their ends scarfed and overlapped to form a substantially continuous insulating structure about the inner edge of the pancake coil. It will be noted that both the insulating structures for the outer and inner edges of pancake coil 16 include at least one angular structure at each of the rounded outer and inner comers. Therefore, in addition to folding an insulator blank to form a channel, the blank must be smoothly formed in an arc to closely conform to its associated corner.

In the prior art, the outer angular channel-like structures such as structure 44, which is shown more clearly in the perspective view of FIG. 2, is formed of pressboard. The depth of the channel and the radius of the curvature of the base of the channel are usually such that softening of the pressboard is essential prior to the molding step. As hereinbefore stated, prior art methods of softening the pressboard all have one or more disadvantages, such as cost, cycle time and reject rate. For example, more costly high rag content pressboard may be resorted to, which allows more severe bending of the insulator blank without rupture than lower cost pressboard, but since the lower cost pressboard is equal in electrical insulating strength to the more costly types, the only thing added to the inductive apparatus is cost.

Further, in order to increase the plasti'cization of the pressboard, water is applied to the pressboard. Soaking the pressboard in water produces a high degree of plasticization, but it greatly extends the forming time. Forming of the insulator blank is accomplished by pressing the blank to shape in the heated dies of a press. Since forming is accomplished at a high pressure, with dies which are heated to a temperature substantially higher than the temperature at which steam will form at atmospheric pressure, premature removal of the structure from the press allows retained moisture to turn to steam, which blisters the part and results in a costly reject. Spraying water on the outer surfaces of the blank reduces the press time, but it only plasticizes the outer surface of the pressboard. Thus, in certain angular, difficult to form shapes, the pressboard may be ruptured at areas of severe bends, which have not been sufficiently plasticized.

Mechanical working of the pressboard to break down interlaminar bonding of the cellulosic fibers, to produce the necessary flexibility, has disadvantages, as the degree of mechanical working necessary requires that the insulation be corrugated deeply, which adversely affects the dimensions of the blank. U.S. Pat. No. 3,351,693, hereinbefore referred to, teaches how the disadvantages of deeply corrugating the material may be overcome, by a method which involves corrugating and then re-corrugating the material. However, it would be desirable, at least in certain instances, to obtain the desired flexibility and plasticity without deeply corrugating the insulating material. Further, the corrugating and re-corrugating method, while performing satisfactorily on the angular channel-like structures for the outer edge of the pancake coil, does not provide the flexibility required for certain of the angular channellike structures for insulating the inner edge of a pancake coil.

In the prior art, the angular channel structures for the inner coil edge, such as the insulating structure 64, which is shown more clearly in the perspective view of FIG. 3, are required to be built up of a plurality of plies of crepe paper which are bonded together in a hot press with a thermally responsive bonding material. These laminated structures are more costly than structures formed of a single piece of pressboard, and it would thus be desirable to be able to construct them of pressboard if some method could be utilized which would form an accurate, well defined structure without voids, and with a low reject rate.

This invention teaches new and improved methods of manufacturing these angular channel-like insulating structures for the inner and outer edges of pancake coils, as Well as other hard to form insulating structures, using less costly grades of insulating material. The structures are accurately formed and well defined without voids or ruptures, with an accuracy which may be economically maintained and repeated from part to part, using a relatively short press cycle time, and with a negligible reject rate.

FIG. 4 is a block diagram which symbolically illustrates the manufacturing steps of a first embodiment of the invention, suitable for insulating structures such as the angular channellike structure 44, shown in FIGS. 1 and 2, for the outer edge of pancake coils. The first step in the method, illustrated by block 70 in FIG. 4, is to cut a blank having predetermined dimensions from a sheet of insulating material having a predetermined thickness. Suitable sheet insulating materials, commonly called pressboard by the electrical industry, are available in thicknesses of 0.0625 to 0.1875 inches. Pressboard is formed of fibrous cellulosic materials, such as wood and cotton.

The size of the blank will depend upon the depth of the channel to be formed, the width of the channel, i.e., the axial dimension or thickness of the pancake coil it is to insulate, and the length of the portion of the edge of the pancake coil it is to insulate. The outer edges of the blank which will form the edges of the channel opposite to the channel base, may be scalloped as shown at 45 in FIG. 2, to allow the structure to be formed with a minimum of wrinkles or folds in the channel legs.

The next step in the method, illustrated by the block 72 in FIG. 4, is to subject the blank to steam, such as by placing the blank in a steam chamber supplied with C. steam at atmospheric pressure. It has been found that steam will rapidly penetrate the pressboard, causing it to expand or swell, and thus break the bonds between the cellulosic fibers which would otherwise resist forming, and which may rupture if formed too severely. Further, steam accomplishes these results so rapidly that the fibers absorb very little moisture. Therefore, the time that the insulator blank is subjected to the steam is critical. The time must be sufficient to uniformly heat the thickness dimension of the material to substantially the temperature of the steam, and then the material should be immediately removed from the steam chamber. Retention of the material in the steam chamber beyond this period causes the pressboard to absorb more moisture, which will extend the press cycle time in the forming step of the manufacturing method. The time in the steam chamber will depend upon the thickness of the pressboard used. For example, using pressboard 0.l25 inch thick, 2 minutes in a steam chamber supplied with 100 C. steam has been found to be excellent.

The time may be reduced somewhat by using steam at higher than atmospheric pressure, which allows its temperature to be increased above 100 C.

The next step in the method, illustrated by block 74 in FIG. 4, is the forming of the blank to the desired shape. The forming step must promptly follow the removal of the blank from the steam chamber, in order to introduce the blank into the press while retaining as much as the temperature to which it has been heated by the steam as possible.

The press for forming the blank has male and female dies of suitable configuration to produce the desired shape, and the dies are preheated to a temperature in the range of 125-150 C. Thus, the hot dies from the hot blank, which has been softened by the steam, to the desired shape. In the prior art, the part is formed essentially cold, even though preheated dies are utilized, since the forming occurs before the heat from the dies has time to penetrate the blank. Thus, in the prior art, the fibers are bent to the new shape, retaining a memory of their original positions, and severe bending may tear or rupture the fibers in highly stressed areas.

The steam heating step taught by this invention, rapidly penetrates the pressboard, causing it to swell and soften the cellulosic fibers, breaking the bonds between the fibers, resulting in an insulator blank which is extremely flexible. This flexible structure is then formed while hot, in heated dies. The cellulosic fibers, instead of being bent to the new shape, are actually rearranged or reoriented into the new shape, with the pressboard giving and stretching without rupture, to conform to the dies. Since very little moisture is absorbed by the blank during the relatively short time that the blank is subjected to steam in the steam chamber, the press cycle time may be relatively short. For example, when using pressboard having a thickness dimension of 0.125 inches, which has been subjected to 100 C. steam for 2 minutes, the press cycle, using dies heated to 125 C., is one minute at a pressure of approximately 800 psi, followed by a gradual reduction in the pressure to approximately psi, over a period of approximately ten seconds, which is followed by maintaining the 10 psi pressure for approximately seconds. The formed part may then be removed from the press. Thus, the complete press cycle time is about one and one-half minutes, which is about onehalf the press cycle time required with prior art methods. The cycle time is reduced because the pressboard is already hot, because there is a minimum amount of moisture to be driven from the pressboard, and because the reorienting of the cellulosic fibers quickly produces a structure having very little memory of its former shape. Thus, the resulting insulating structure has a smooth surface, free of tears or ruptures, and it has accurate dimensions which are maintained without a tendency of the channel to subsequently open up and assume its former shape. Still further, the reduction in the press time reduces the manufacturing cost for each structure, and the method allows less costly grates of pressboard to be utilized.

FIG. 5 is a block diagram which symbolically illustrates the steps of a method which follows the teachings of another embodiment of the invention. When the insulating structure to be formed requires extreme bending and stretching, such as the inside edge angular channel-like insulating structure 64, shown in FIGS. 1 and 3, which is difiicult to form due to the deep channel and relatively small radius of the channel base, an additional mechanical working step will prepare the pressboard for the forming step.

More specifically, the first step in this embodiment of the invention, illustrated in FIG. 5 with block 80, is to cut a blank to the desired configuration from a sheet of insulating material of predetermined thickness, such as pressboard, and is similar to the like step 70 in the method shown in FIG. 4. Similar to the blank used for the outside corner insulating structure 44, the blank used for the inside corner insulating structure 64 may have scalloped edges 65 to facilitate stretching the outer edges of the channel.

The next step, illustrated by block 82 in FIG. 5, is to subject the blank to steam, for example in a steam chamber supplied with 100 C. steam, such as hereinbefore described relative to step 72 in the method of FIG. 4. However, instead of forming the blank after removal of the blank from the steam chamber,

the next step, which is illustrated by block 84 in FIG. 5, is to promptly corrugate the blank in corrugating rolls immediately upon its removal from the steam chamber. The whole blank may be corrugated, or just a portion or portions which will be subjected to the greatest rearrangement of the fibers during the forming step.

Corrugating immediately following the removal of the blank from the steam chamber, while the blank still retains substantially all of the heat imparted thereto by the steam, unlocks the fibers and allows them to be stretched or compressed during forming without tearing, folding or wrinkling.

In the prior art, corrugating a blank to mechanically work and soften the pressboard is accomplished cold, which requires that the pressboard be deeply corrugated in order to obtain the necessary flexibility. Deeply corrugating the material substantially changes its dimensions, which results in a formed part having unpredictable dimensions, unless an additional lighter corrugating step is added, wherein the material is corrugated at an angle to the first corrugating direction, to return the material to substantially its original dimensions.

Corrugating the insulator blank immediately following the removal of the blank from the steam chamber, will unlock the cellulosic fibers while utilizing a very light corrugation. Thus, the blank dimensions are substantially unafiected by the corrugating step, making it unnecessary to add an additional corrugating step. For example, the corrugating step 84 need only be sufficient to place indentations in the blank which results in a permanent deformation having a depth of approxi-mately one thirty-second of an inch, which corrugations are shown generally by lines 67 in the insulating structure 64 shown in FIG. 3.

The next step, after the corrugating step 84, is to again place.

the blank in a steam chamber, which step is illustrated in FIG. 5 by block 86, where the blank is again subjected to C. steam for approximately the same length of time as the first heatingstep 82. For example, the second steam heating step may be approximately 2 minutes in length for 0.125 inch thick pressboard. Upon removal of the blank from the steam chamber, it is immediately placed in the heated dies of a forming press, and formed to the desired shape, as illustrated by block 88 of FIG. 5, which is similar to the step hereinbefore described relative to the forming method of FIG. 4.

In summary, there has been disclosed new and improved methods of manufacturing difficult to form insulating structures for electrical inductive apparatus, such as the angular, channel-like insulating structures for the outer and inner edges of pancake coils. The methods disclosed not only reduce the manufacturing time required to produce the structures, but provide structures which are free of tears and ruptures, and which retain their as formed shape. The disclosed methods also allow pressboard to be used for structures which in the prior art require a laminated structure, due to the impracticability of forming the pressboard to such shapes with prior art methods. Further, the disclosed methods allow less costly grades of pressboard to be utilized, which, along with the reduction in manufacturing time, produce a better structure at lower cost.

Since numerous changes may be made in the abovedescribed apparatus and different embodiments of the invention may be made without departing from the spirit thereof, it is intended that all matter contained in the foregoing description, or shown in the accompany drawings, shall be interpreted as illustrative and not in a limiting sense.

We claim: 1. A method of constructing rigid, angular, channel-shaped insulating structures from low rag content pressboard for tightly fitting the inner edges of pancake coils associated with electrical inductive apparatus, comprising the steps of:

cutting a blank having predetermined dimensions from a sheet of pressboard of predetermined thickness,

subjecting said blank to steam to heat and moisturize the blank, for a period of time not substantially longer than required to uniformly heat the blank through its thickness dimension,

insulating structures from low rag content pressboard for promptly corrugating at least predetermined portion of said blank, while it still retains substantially the temperature imparted to it by the steam,

subjecting said blank to steam for a second time, to heat and moisturize the blank, not substantially longer than required to uniformly heat the blank through its thickness dimension,

and then promptly forming said blank to a predetermined angular, channel shaped configuration for tightly fitting the inside edge of a pancake type coil, while it still retains substantially the temperature imparted to it by the steam, said forming step including placing said blank in a press having heated dies, pressing said blank in the heated dies at a first pressure for a predetermined period of time, reducing the pressure over a predetermined period of 15 time to a second pressure, and maintaining the second pressure for a predetermined period of time.

2. A method of constructing rigid, angular, channel shaped tightly fitting the inner edges of pancake coils associated with electrical inductive apparatus, comprising the steps of:

cutting a blank having predetermined dimensions from a sheet of pressboard having a thickness dimension of about 0.125 inch, subjecting said blank to steam to heat and moisturize the blank, for at least, but not substantially longer than 2 minutes, promptly corrugating at least a predetermined portion of said blank, while it still retains substantially the temperature imparted to it by the steam, subjecting said blank to steam for a second time to heat and moisturize the blank, for at least, but not substantially longer than two minutes, and then promptly forming said blank to a predetermined angular, channel-shaped configuration for tightly fitting the inside edge of a pancake type coil, while it still retains substantially the temperature imparted to it by the steam.

I 0' i i 

1. A method of constructing rigid, angular, channel-shaped insulating structures from low rag content pressboard for tightly fitting the inner edges of pancake coils associated with electrical inductive apparatus, comprising the steps of: cutting a blank having predetermined dimensions from a sheet of pressboard of predetermined thickness, subjecting said blank to steam to heat and moisturize the blank, for a period of time not substantially longer than required to uniformly heat the blank through its thickness dimension, promptly corrugating at least predetermined portion of said blank, while it still retains substantially the temperature imparted to it by the steam, subjecting said blank to steam for a second time, to heat and moisturize the blank, not substantially longer than required to uniformly heat the blank through its thickness dimension, and then promptly forming said blank to a predetermined angular, channel shaped configuration for tightly fitting the inside edge of a pancake type coil, while it still retains substantially the temperature imparted to it by the steam, said forming step including placing said blank in a press having heated dies, pressing said blank in the heated dies at a first pressure for a predetermined period of time, reducing the pressure over a predetermined period of time to a second pressure, and maintaining the second pressure for a predetermined period of time.
 2. A method of constructing rigid, angular, channel shaped insulating structures from low rag content pressboard for tightly fitting the inner edges of pancake coils associated with electrical inductive apparatus, comprising the steps of: cutting a blank having predetermined dimensions from a sheet of pressboard having a thickness dimension of about 0.125 inch, subjecting said blank to steam to heat and moisturize the blank, for at least, but not substantially longer than 2 minutes, promptly corrugating at least a predetermined portion of said blank, while it still retains substantially the temperature imparted to it by the steam, subjecting said blank to steam for a second time to heat and moisturize the blank, fOr at least, but not substantially longer than two minutes, and then promptly forming said blank to a predetermined angular, channel-shaped configuration for tightly fitting the inside edge of a pancake type coil, while it still retains substantially the temperature imparted to it by the steam. 